1. Field of the Invention
The present invention relates to a rotary-anode type X-ray tube and, more particularly, to an improvement in the structure of a bearing for supporting a rotary-anode type X-ray tube.
2. Description of the Related Art
As is known, in a rotary-anode type X-ray tube, a disk-like anode target is supported by a rotary structure and a stationary shaft which have a bearing portion therebetween, and an electron beam emitted from a cathode is radiated on the anode target while the anode target is rotated at a high speed by energizing an electromagnetic coil arranged outside a vacuum envelope, thus irradiating X-rays. The bearing portion is constituted by a roller bearing, such as a ball bearing, or a hydrodynamic pressure type sliding bearing which has bearing surfaces with spiral grooves and uses a metal lubricant consisting of, e.g., gallium (Ga) or a gallium-, indiumtin (Ga-In-Sn) alloy, which is liquified during an operation. Rotary-anode type X-ray tubes using the latter bearing are disclosed in, e.g., Published Examined Japanese Patent Application No. 60-21463 and Published Unexamined Japanese Patent Application Nos. 60-97536, 60-117531, 62-287555, 2-227947, and 2-227948.
In the rotary-anode type X-ray tubes disclosed in the above-mentioned official gazettes, a liquid metal lubricant consisting of Ga or a Ga-alloy is applied between the bearing surfaces of the sliding bearing. In this arrangement, however, when a tube is processed at a high temperature in the process of manufacturing an X-ray tube, or the tube is heated to a high temperature due to heat generated during an operation of the X-ray tube, mutual penetration may occur between a metal constituting these bearing surfaces and the lubricant, resulting in a gradual decrease in the amount of liquid metal lubricant. This may damage the bearing surfaces. As a result, the sliding bearing may not be stably operated for a long period of time.